The present invention pertains to a variable heat transfer and adjustable holddown load device for a double backer for the production of corrugated paperboard in which the conventional driven web holddown belt is eliminated.
In a typical prior art double backer, a liner web is brought into contact with the glued flute tips of a single face corrugated web, and the freshly glued double face web is then passed over the surfaces of a number of serially arranged heating units, usually steam chests, to cause the starch-based glue to set and to drive moisture from the web. Double face web travel over flat heated surfaces of steam chests is typically provided by a wide driven holddown belt in direct contact with the upper face of corrugated web. The top face of the belt, in turn, is held in contact with the traveling web by any of several types of weight or force applying devices, well known in prior art. For example, the holddown belt may be engaged by a series of weighted ballast rollers, it may be forced into contact with the web by air pressure from a system of nozzles located over the web, or an arrangement of inflatable air bladders may be operated to press the moving holddown belt into contact with the double face web. It is also known to provide means for varying the ballast load applied to the holddown belt and web, both longitudinally in the machine direction and laterally in the cross machine direction.
The use of a driven holddown belt in a double backer has a number of attendant disadvantages. The holddown belt must be mounted for continuous travel in the manner of the conventional continuous conveyor belt system and, therefore, must also include a separate belt drive means. The holddown belt also must necessarily overlie the entire surface of corrugated web, at least in the heating section, and, as a result, may inhibit the escape of moisture from the board as it dries. Also, the edges of the belt which overhang the edges of the corrugated web run on the heating surfaces and are subject to wear.
It has also been found that, in the production of double wall corrugated paperboard, heat transfer from the underlying heating units to the freshly glued flute tips of the upper single face web is often irregular or insufficient to provide adequate green bond strength. As a result, the lower liner web may readily reach adequate green bond strength, but the bond between the upper single face web and the intermediate liner in the double wall board will be inadequate.